The present invention relates to support structures comprising tensile support bands, particularly such structures for retaining concentric vessels, and more particularly such structures with low thermal conductivity for supporting closely spaced concentric vessels in cryogenic applications.
In systems such as cryostats for holding superconducting magnets for magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR), or for space-borne dewars, it is necessary to firmly mechanically support concentric hollow vessels, in such a way that the support structure conducts very little heat. Conventional arrangements include the use of fibre reinforced racetrack-shaped tensile support bands. The term ‘racetrack-shaped’ is understood in the art to refer to a loop shape comprising two parallel sides joined by semicircular end portions. Such tensile support bands are held by mounting points on the outer and inner vessels. The tensile support bands are put under tension and the inner vessel is thereby supported within the outer vessel, but not in contact with it other than through the tensile support bands. By making the tensile support bands of fibre-reinforced composite materials, they may be made to have a very high tensile strength, and a very low thermal conductivity. Tensile support bands made from composite materials containing fibres such as glass or carbon offer unique strength, stiffness, and thermal properties compared with other materials. However, the mounting points that allow the tensile support band to be connected to the vessels are bulky and complicated relative to the tensile support band itself. The result is that the potential space saving advantages of the high strength composite cannot be fully exploited in many applications, since the minimum separation between concentric vessels becomes defined by the need to accommodate the mounting points for tensile support between the concentric vessels.
The high strength and stiffness, and advantageous thermal properties of glass, carbon or other fibre reinforced plastics are widely utilised for load bearing tensile support bands. Such tensile support bands are typically manufactured using a filament winding process to achieve the best strength of the fibres, commonly in the form of racetrack shaped tensile support bands. In order to achieve the highest tensile strength, the geometry of the tensile support bands at the loop ends must be maintained within certain parameters, in particular the diameter to thickness ratio (D/t) must be kept high (typically >10), leading to a cross section of high aspect ratio (W/t) (typically width to thickness ratio >10). Such tensile support bands are sensitive to bending loads, which result in severely reduced tensile strength. In practical applications, where misalignment and movements of the supported structures occur, the ends of the tensile support bands must be supported using a suitable, typically spherical, bearing arrangement.
Examples of arrangements using tensile composite tensile support bands are described by R. Kevin Giesy in Cryogenic Engineering Conference/International Cryogenic Materials Conference Jul. 17-21, 1995, Columbus Ohio and by R. P. Reed and M. Golda in Cryogenics 37 (1997) pages. 233-250.
FIGS. 1A and 1B illustrate a conventional mounting point 10 for a tensile support band. The minimum height H of the mounting point is largely determined by the need to provide a bearing arrangement. A support pin 12 is carried by a generally U-shaped support 14, shown cut away in FIG. 1A. The support 14 is itself rigidly mechanically attached, for example by welding, to one of the vessels 15 between which the tensile support band 20 is to be installed. A bearing 16 of generally spherical shape is provided on the support pin 12. A support roller 18 is provided, having an inner surface complementary to the bearing 16, and an outer surface of cylindrical form. The tensile support band 20 itself is placed over, and in contact with, the cylindrical outer surface of the support roller 18. The support roller is free to move, within a limited range, by rotation about the bearing 16. This allows effective retention of the tensile support band 20 even in the case of misalignments and movement, avoiding placing uneven loading on the tensile support band, which might otherwise occur if such a mounting point were not provided.
The height H of the mounting point could possibly be reduced by reducing the loop diameter D of the tensile support band. However, in order to preserve the ratio D/t>10, the thickness t of the tensile support band would have to be reduced by the same proportion. This, in turn, would require an increase in the width W of the tensile support band to maintain the required tensile support band strength. The reduced diameter D and increased width W would cause the tensile support band to be even more susceptible to damage by bending loads due to misalignment or movement. Changing the orientation of the tensile support band so that the pin 12 were mounted perpendicular to the surface of the vessel 15 would typically riot reduce the height H since the width W of the tensile support band is typically greater than its diameter D.
FIG. 2 illustrates a typical conventional tensile support band 20. The diameter D is reduced as far as possible, to allow the height H of the mounting point 10 to be minimised. In order to maintain the preferred ratio D/t>10, the thickness t is also minimised. In order to maintain a required tensile strength despite such reduced thickness, the width W is increased.
The combination of high aspect ratio W/t cross-section and the need to accommodate a bearing 16 results in a relatively large and complex system in which the size of the tensile support band mounting points 10 exceed the rest of the tensile support band 20. In many applications the space available for installation of tensile support bands and their associated mounting points is limited, and minimising the space required is an important design requirement, which is thus compromised. This is particularly applicable to the suspension of close-fitting concentric vessels such as are used in super-conducting magnet cryostats. In addition, the cost of the bearing assembly of mounting point 10 is relatively high.
The present invention provides an arrangement which aims to alleviate at least some of the drawbacks of the prior art. The present invention accordingly provides methods and/or apparatus as defined in the appended claims.